An important operational consideration for many, if not all, carbon dioxide processing systems is throughput, which may be defined as the amount of articles or materials that may be processed in a given period of time. In various processes that utilize carbon dioxide as a solvent, multiple solvent baths may be used to improve operational efficiency by increasing throughput. For example, in a carbon dioxide based dry cleaning system, it may be desirable to provide pre-washing, washing, rinsing, and distillation operations. The pre-wash, wash, and rinse solutions may each have a different chemical composition. If a single tank were used to store the various baths, the composition of the solution in the tank would need to be adjusted between each operation, which may increase cycle time and decrease throughput. By utilizing a tank for storing a pre-wash solution, a separate tank for storing wash solution, another separate tank for storing a rinse solution, and yet another separate distillation tank, the dry cleaning system may be able to cycle through the operations of pre-washing, washing, rinsing, and distillation in a more timely manner.
U.S. Pat. No. 5,267,455 to Dewees et al. proposes a dry cleaning system particularly suited for employing supercritical carbon dioxide as the cleaning fluid consisting of a sealable cleaning vessel containing a rotatable drum adapted for holding soiled substrate, a cleaning fluid storage vessel, and a separate gas vaporizer vessel for recycling used cleaning fluid.
While utilizing multiple solvent baths in carbon dioxide based systems may improve operational efficiency, the increased capital costs associated with such systems may outweigh some of the operational benefits. Carbon dioxide based systems may be required to operate at elevated pressures in order to maintain the carbon dioxide solvent in either a liquid or a supercritical state. As a result, each of the separate tanks must be a stamp coded and certified pressure vessel, which can be quite expensive. Additionally, each of the separate tanks must be in constant fluid communication with a pressure relief valve. Thus, the capital cost of constructing a carbon dioxide based system having multiple solvent baths may outweigh some, if not all, of the operational benefits that may be gained by utilizing such a system.
WO 97/33031 proposes a storage/still vessel that both distills and stores the liquid gas for use in another cleaning cycle. The storage/still vessel includes a tube located within an inner cavity of an outer tank. At the top of the tube is an opening that provides fluid communication between the inner cavity and an inner chamber of the tube. The bottom of the tube has a one-way control valve that allows the liquid gas to flow from the inner cavity to the inner chamber. The vessel has a heating element that can heat and vaporize the liquid gas within the inner chamber of the tube. The still vaporizes the liquid gas within the tube, which causes liquid to flow from the inner cavity to the inner chamber. Vaporized liquid gas condenses and falls into the inner cavity, which then forms a layer of distilled “clean” liquid, on top of a layer of undistilled “dirty” liquid. The cycle is continued until the level of dirty liquid falls below the one way control valve.
WO 97/33031 does not provide an apparatus utilizing a carbon dioxide based solvent that will allow two or more fluids having different chemical compositions to be stored and/or processed within the same pressure vessel while maintaining the different chemical compositions of the two or more fluids.